Continuous field based upper bound analysis for three-dimensional tunnel face stability in undrained clay

A new numerical upper-bound method applicable to three-dimensional stability problems in undrained clay is proposed and applied in the analysis of tunnel face stability. Comparisons with the elasto-plastic finite element method (FEM) reveal that this method could provide sufficiently accurate upper-bound prediction for the tunnel face stability factors. For tunnels with large cover depth, the obtained results greatly improve the existing upper bound solutions. Their corresponding velocity fields are investigated, which show a similar evolving pattern with those of the FEM. A discussion is made to compare the various advantages of multi-block mechanisms and continuous mechanisms in tunnel face stability.